Levi, Dennis M. PROJECT SUMMARY/ABSTRACT Amblyopia is a developmental disorder of spatial vision usually associated with the presence of strabismus, anisometropia or form deprivation early in life. It affects visual acuity, contrast sensitivity and position acuity. Amblyopia is clinically important because, aside from refractive error, it is the most frequent cause of vision loss in infants and young children, and amblyopia is of basic interest because it reflects the neural impairment which can occur when normal visual development is disrupted. The long-range objectives of this research are to understand the mechanisms that limit spatial vision in humans with amblyopia, with the ultimate goal of developing new approaches aimed at more effective treatment. While our focus in past studies has been on the sensory losses, here we propose the application of a number of innovative methods that will enable us to test specific hypotheses about the role of eye-movements in limiting spatial vision in amblyopia. Aim 1 tests the hypothesis that abnormal spatial vision is at least in part a consequence of fixational eye movements. Specifically we ask how both the retinal image motion induced by these abnormal fixational eye movements, and the eye movements per se, influence visual resolution, visual acuity, crowding and the precision and accuracy of spatial localization. We will use a high-resolution video eye tracker, the Tracking Scanning Laser Ophthalmoscope (TSLO) which allows stimuli to be delivered to precisely identifiable locations on the retina, to test our hypothesis using three different approaches: i) We will measure, model and characterize fixational eye movements in normal and amblyopic adults while performing visual tasks that are compromised in amblyopia. ii) We will assess the role of retinal image motion by comparing visual performance on these tasks under stabilized, unstabilized and ?playback? (i.e., superimposing the amblyopic eye's retinal image motion on the nonamblyopic eye) conditions. iii) We will assess the role of eye movements per se by evaluating visual performance around the time of occurrence of drifts and microsaccades. Compared to normal observers, strabismic amblyopes have significantly longer saccadic and manual latencies to stimuli seen with their amblyopic eyes, compared to their fellow eyes. Aim 2 tests the hypothesis that for strabismic amblyopes, the frequent microsaccades and accompanying saccadic suppression and attentional shifts made while strabismics struggle to maintain fixation with their amblyopic eyes, result in all types of reaction times being irreducibly delayed, commensurate with the degree of unsteadiness. Because the degree of unsteadiness is correlated with LogMAR acuity, it follows that reaction time and saccadic latency are also correlated with LogMAR acuity. Aim 2 tests a number of specific hypotheses regarding the role of microsaccades in reaction time and visual search. 1